Production of an aromatic polycarbonate by polycondensation of an aromatic dihydroxy compound and a carbonic acid diester, so-called transesterification is not only relatively simple as compared with a phosgene method (interfacial polymerization method) and is superior in view of operation and cost, but also revaluated in recent years from the viewpoint of environmental protection since no halogen type solvent such as phosgene or methylene chloride which is highly toxic is used.
However, the transesterification has several drawbacks in view of physical properties as compared with the phosgene method, and thus it is scarcely employed as a large scale industrial process. A polycarbonate obtained by the transesterification, as a representative example, has such problems that branches are likely to form, fluidity and moldability tend to decrease, and it is poor in hue of the product, as compared with a product obtained by the phosgene method.
Several proposals have been made to overcome these problems. For example, a method for obtaining an aromatic polycarbonate excellent in hue, hue stability and heat stability at the time of molding, by making the contents of an isomer and a derivative in the aromatic dihydroxy compound used as a material to be within specific ranges (JP-A-8-104747, JP-A-8-104748, JP-A-8-277236), a method for obtaining a polycarbonate having less branches, by making the phenoxybenzoic acid content in the main chain to be a specific amount with a specific catalyst under a specific reaction condition (JP-A-7-18069, JP-A-8-109251, JP-A-9-278877), or a method for obtaining a less colored polycarbonate by making the p-hydroxyacetophenone content to be at most a specific amount (JP-A-8-27263) may, for example, be mentioned. Such methods are methods proposed to overcome conventional problems and to decrease formation of branches and to improve the hue.
On the other hand, a commonly employed aromatic polycarbonate has a linear molecular structure, and a polycarbonate having such a molecular structure may be poor in e.g. melt elasticity and melt strength at the time of melt molding, and their improvements have been desired. Actually, as a method for improving melt properties such as melt elasticity and melt strength of such a polycarbonate, a method for branching a polycarbonate by an interfacial polymerization method by using a polyfunctional compound such as 1,1,1-tris(4-hydroxyphenyl)ethane (THPE) or 1,3,5-tris(4-hydroxyphenyl)benzene as a branching agent together with 2,2-bis(4-hydroxydiphenyl)propane (hereinafter referred to simply as “bisphenol A” or “BPA”) has conventionally been known (JP-B-44-17149, JP-B-47-2918, JP-A-2-55725, JP-A-4-89824).
However, an aromatic polycarbonate has a high melt viscosity even if it has a linear structure, and further, a polycarbonate having a branched structure introduced by copolymerization with a polyfunctional compound has a higher melt viscosity and a decreased fluidity. Of such a polycarbonate having a high melt viscosity and being poor in fluidity, the molding conditions may be limited, or molding unevenness occurs, whereby it is difficult to stably obtain a uniform molded product.
In order to overcome these problems, various attempts have been done on a melt polymerization method employing a carboxylic acid diester and an aromatic dihydroxy compound, however, a branching agent may undergo e.g. decomposition at a high temperature, whereby no effect of branching is obtained, and further, it may cause coloring, and accordingly no satisfactory results have been obtained (JP-A-4-89824, JP-A-6-136112, JP-B-7-37517, JP-B-7-116285).
The present invention is to provide a method for producing a branched aromatic polycarbonate excellent in quality. More particularly, it is to provide a branched aromatic polycarbonate having improved melt properties and further having improved hue and residence stability, and a method for producing such a branched aromatic polycarbonate efficiently.